One of the major challenges facing petroleum refiners today is the ultra-deep desulfurization of diesel, which requires that sulfur levels be reduced to less than 10 ppm. While the concentrations of thiophenes and, to a lesser extent, benzothiophenes can be reduced to the required levels by catalytic hydrodesulfurization, removal of sulfur from 4,6-dialkyl dibenzothiophenes to a similar extent is extremely difficult because the alkyl groups inhibit access to the sulfur atom. A further complication is that the hydrogen demand for removing sulfur from dialkyl dibenzothiophenes is greater than that from other sulfur-containing molecules because one of the benzene rings must first undergo hydrogenation before desulfurization can occur. Furthermore, at the high hydrogen pressures required for desulfurization of dialkyl dibenzothiophenes, some of the aromatic compounds present in diesel also undergo hydrogenation, further raising the overall hydrogen required for deep desulfurization.
A possible alternative to hydrodesulfurization is selective adsorption of thiophene derivatives on a solid adsorbent. The most promising of materials that have been explored to date are based on cation-exchanged zeolites and metals, metal halides, other metal salts supported on activated carbon (AC). Metal cations such as Na+, K+, Ag+, Ni2+, Cu2+, Zn2+, Pd2+, Fe3+, Ce3+ have been considered as adsorption centers. Adsorbents based on activated carbon have generally been found to exhibit higher adsorption capacities than those based on zeolites, and it has been reported that the adsorption capacity for benzothiophene and dibenzothiophene follows the order Ag+/AC>Ni2+/AC>Cu2+/AC>Zn2+/AC>AC>Fe3+/AC. Amongst zeolitic materials, Cu+/Y has been reported to be the most effective. What has not been addressed in these studies is the relative strength of binding of arenes such as naphthalene, anthracene, and their derivative compared to benzothiophene, dibenzothiophene, and other thiophene derivatives. This is an important issue, since liquid fuels, such as hydrotreated diesel contains much higher concentrations of arenes (often more than 10%) than of thiophene derivatives (often less than 50 ppm) following deep hydro-desulfurization.
There is still a need for improved methods to select metal atoms or cations that will preferentially bind, for example, dimethyl dibenzothiophene with respect to anthracene or naphthalene. The invention relates to a method to predict and select metal adsorption centers for preferential adsorption of thiophene derivatives in the presence of arenes.